1. Technical Field
The invention relates to a small wide-angle lens system, and particularly relates to: an imaging lens that is suitable for a variety of digital input devices employing a solid-state imaging element such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS)—for example, a digital camera, a television phone, and a surveillance camera such as a vehicle-mounting camera and a door phone; and an imaging device that converts an optical image formed by the imaging lens into imaging signals.
2. Description of the Related Art
For example, in surveillance cameras or vehicle-mounting cameras, a wide-angle type imaging lens having a wide viewing angle is used to taking an image with a wide area. JP 2002-244031 A, JP 2005-227426 A and JP 2006-292988 A (corresponding to US 2006/0227434 A) has proposed, as such a wide-angle lens, lenses having a smaller size than that of the conventional eight-lens-group wide-angle lens, by constructing the lens system with 4 lenses in total.
In recent years, the wide-angle lens for use in the digital input devices is required a long back focus, a small size, and a small image distortion. However, the wide-angle lenses described in the above publications are not sufficient in downsizing. The total length of those lenses (distance from a vertex of a first lens to an image formation surface) is large, and the diameter of the first lens increases together with the total length thereof. Also, in order to achieve wide angle, those lens have a lens configuration similar to a fisheye type. Therefore, the distortion thereof is large and a greatly distorted image is formed. Accordingly, there is a demand for development of a lens that has a wide viewing angle, a small size, and a small image distortion.